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8892 



Bureau of Mines Information Circular/1982 



A Feasibility Study of the Use of Surface 
Redox Measurements To Detect 
Subsurface Methane, Coal Burns, 
and Hydrothermal Deposits 

By Richard G. Burdick and Terry L. Radich 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8892 

A Feasibility Study of the Use of Surface 
Redox Measurements To Detect 
Subsurface Methane, Coal Burns, 
and Hydrothermal Deposits 

By Richard G. Burdick and Terry L. Radich 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 







This publication has been cataloged as follows: 



Burdick, Richard G 

A feasibility study of the use of surface redox measurements to 
detect subsurface methane, coal burns, and hydrothermal deposits. 



(Information circular ; 8892) 
Includes bibliographical references. 
Supt. of Docs, no.: 1 28.27:889^. 
1. Mine safety. 2. Soils — Testing. 



•5. Oxidation-retkict ion reac- 



tion. 4. Prospecting— Geophysical methods. I. Radich, I'erry ].. !l. 
Title. III. Series: Information circular (llnited Slates. Bureau of 
Mines) : 889 2. 



TN^^&A^A- 622s I622'.028'7j 82-600222 






<i 



u 



CONTENTS 



Abstract 1 

Introduction 2 

Acknowledgments 2 

<ij Basis for use of redox method 2 

"^ Description of redox probe 4 

Field considerations for obtaining test data 6 

Experimental field results 7 

0- Case 1 7 

Results, site A 8 

Results, site B 9 

Case 2 9 

Case 3 12 

Conclusions 13 

Appendix 14 

ILLUSTRATIONS 

1. Comparison between laboratory and field redox measurements 4 

2. Single-electrode, 1-1/2-ln-dlam probe 5 

3. Double-electrode, 4-ln-dlam probe 5 

4. Graph showing data readings plotted against time 6 

5. Auger and bits used In field tests 7 

6. Field Installation of probe 7 

7 . SUFCo site A showing coal seam and burn area 8 

8. Field curve of SUFCo site A 9 

9. Field curve of SUFCo site B 10 

10. 3-D computer model of Bear Mine 11 

11. Field curve, Bear Mine site 11 

12. Plat of Hazel A veins 12 

13. Field curve. Hazel A Mine, line A 12 

14. Field curve, Hazel A Mine, line B 12 

A-1. Construction details of single-electrode probe 14 

A-2. Construction details of double-electrode probe 15 




^ 



A FEASIBILITY STUDY OF THE USE OF SURFACE REDOX MEASUREMENTS 

TO DETECT SUBSURFACE METHANE, COAL BURNS, 

AND HYDROTHERMAL DEPOSITS 

By Richard G. Burdick ' and Terry L. Radich^ 



ABSTRACT 

The Bureau of Mines conducted this research to determine the feasi- 
bility of using soil redox measurements, a relatively new method in 
mining geophysics, to locate or define a variety of conditions that 
are hazardous or otherwise of interest to the mining industry. The 
method was used in three widely separated areas in an attempt to de- 
fine subsurface concentrations of methane, the edges of coal-burn 
areas, and a hydrothermal vein set. Both a description of the method 
used and test cases are given in this report. 

^Engineering technician. 
''Electronics technician. 
Both authors are with the Denver Research Center, Bureau of Mines, Denver, Colo. 



INTRODUCTION 



The concept of using geophysics for 
premining investigations is based upon 
the ability to locate or delineate haz- 
ards or other areas of interest to the 
mining community, so that their effects 
may be considered during the advance 
planning phases of the proposed mining. 

Despite the rapid growth of the appli- 
cation of geophysics in the mining in- 
dustry during the past decade, many prob- 
lems remain that are not readily solved 
by current geophysical methods. The 
oxidation-reduction potential (redox) 
method, one of the less commonly used 
methods, was selected as having potential 
for solving some of these problems. 

The redox method is generally thought 
of as a laboratory means to determine the 
electrochemical balance of a chemical 
reaction. Its use outside the laboratory 
and in a geologic environment is not com- 
monly considered. However, previous 
work, predominant in the field of well- 
logging, has shown that the method is 



suitable in a geologic setting as well as 
in the laboratory. 3 

The main value of the redox method in 
mining environments is its use as a rapid 
reconnaissance tool for locating such 
diverse geochemical anomalies as methane 
pockets, coal-burn areas, hydrothermal 
deposits, or other phenomena that may 
cause a chemical reaction in the overly- 
ing rocks. For this purpose, a traverse 
or grid of shallow holes allows many 
readings to be made, from which the ex- 
tent of the anomaly may be interpreted. 

Two different types of hazards were 
originally chosen for this research — 
methane pockets and underground coal 
burns. It was later decided to test the 
redox method's response to vein-type 
deposits because of their interest to 
parts of the mining industry. This re- 
port describes the brief feasibility 
tests performed in three different areas 
and gives the test results and their cor- 
relation with known conditions. 



ACKNOWLEDGMENTS 



The authors wish to express their 
thanks to the SUFCo No. 1 Mine, the Bear 
Mine, and the Hazel A Mine, for per- 
mission to work on their properties and 
for details on the various geologic 
and mining problems in each area 
that were necessary in performing these 



investigations. In addition, we would 
like to thank Kerry Frame and Dall Dimick 
of SUFCo for giving of their time to 
advise and assist in the fieldwork at 
their mine, and we would like to thank 
Bill Bear and Dave Herr of the Bear Mine 
for their help and encouragement. 



BASIS FOR USE OF REDOX METHOD 



Redox measurements are analogous to 
using earth materials as a half-cell. 
The other part of the cell, or 
electrochemical couple, is a satu- 
rated calomel, or mercury -mercury 
chloride (Hg-Hg2Cl2), half -cell of 
known potential placed in contact 
with with the soil through a porous 
membrane. The algebraic difference 
of the soil-calomel half-cells is 
measured with a sensitive voltmeter; 
the result is termed the redox poten- 
tial of the soil. As the calomel cell 
value is a constant, differences in 
the measured values are attributable 



to differences in the soil half-cell 
potentials. 

■^Bacon, L. , and E. Charles. Redox Rem- 
anent Magnetization. Geophys./ v. 46, 
No. 8, August 1981, pp. 1168-1181. 

Colombo, U. Differential Electric 
Log. Geophys. Prospecting, v. 1, No. 7, 
1959. 

Karaoguz, D. An Experimental Study of 
Redox Logging. Log Analyst, May-June 
1970. 

Pirson, S. J. Redox Log Interprets 
Reservoir Potential. Oil and Gas J. , 
v. 66, No. 27, July 1968, pp. 69-75. 



The term "soil," as used above, refers 
to the upper few feet of surface materi- 
als composed of clays, silica particles, 
and various amounts of different minerals 
that are susceptible to reversible 
oxidation-reduction reactions. Of these, 
the clay minerals are probably the most 
prevalent and probably contribute the 
most to the earth half-cell potential. 
The clay micelles range from a maximum of 
2 ym to colloidal in size and are plate- 
like in shape, which gives them a very 
large surface area in relation to their 
size. These clay micelles are capa- 
ble of holding both cations and anions 
on their surfaces, which allows them to 
exhibit a relatively high pseudo-chemical 
activity. 

Soil, by definiation, is the result of 
weathering of rocks. Because of this 
origin it may be considered to be of 
reasonably uniform composition over 
short distances and is assumed to have 
been subjected to reasonably uniform 
weathering processes. From this it is 
assumed that the redox potential should 
not vary significantly over short 
lateral distances. This has been found 
to be the case during the Bureau's field 
studies. 

If, after formation or emplacement of 
the soil, the soil is exposed to differ- 
ent cations or anions that attach to the 
clay micelles, or if a chemical reaction 
changes the valence state of the already 
attached ions, a change in the redox 
potential will occur. Field measurements 
cannot determine what mechanism has taken 
place, or if a change in potential has 
occurred. They can only determine what 
redox potential exists at that point at 
the time of measurement. 

The above explanation has dealt with 
the major contributor to the soil redox 
system, clay. Small silica particles 
exhibit a similar, though less active, 
role in the system, while other minerals 
may exhibit similar phenomena or may 
actually combine chemically with the var- 
ious reactants. However, compared with 
clay and fine silica, their contribution 



to the redox potential may be considered 
negligible in most cases. 

The source of the reactants that can 
change the redox potential may be gases 
formed from combustion of coal, ions 
transported by hydrothermal action, hy- 
drocarbon vapors and gases, or reactants 
from surface sources. In addition, sul- 
fide deposits adjacent to other rocks can 
cause large redox potentials.'^ 

When using the redox method for locat- 
ing anomalous subsurface phenomena, the 
following assumptions are made. The sur- 
face soil-rock materials are composed of 
clays, silica particles, and various 
amounts of different minerals that are 
subject to reversible oxidation-reduction 
reactions. 5 The reactants involved in 
the subsurface phenomena of interest are 
capable of migration to the surface. And 
finally, the oxidation or reduction 
caused by the react ant is not subse- 
quently altered by other reactions. This 
oxidation or reduction may take place 
over a period of months or years, or in 
some cases even centuries, as the 
oxidant-reductant may only consist of a 
few molecules per year percolating upward 
through many feet of rock. If the react- 
ant material comes from a localized 
source, such as a methane pocket or a 
burned area in coal, the redox potential 
of the rocks above will also show a 
localization effect if the reactant move- 
ment is vertical. In the case of coal 
burns, the effect may be accelerated by 
gas movement along cracks or fissures in 
the overburden. 

These basic assumptions must be kept in 
mind when performing the tests and making 
interpretations of the data. There is 
not an extensive background of data or 
results for the surface redox method as 
applied to mining problems. Therefore, 
the method and the interpretations should 
be correlated with known information 
about the anomalous conditions as fully 
as possible. 

^First work cited in footnote 3. 
^Fourth work cited in footnote 3. 



DESCRIPTION OF REDOX PROBE 



The redox probe is a ruggedized version 
of a typical laboratory measurement sys- 
tem in which a saturated calomel (Hg- 
Hg2Cl2) cell with a known potential of 
-0.2415 V is used as a reference standard 
against which the soil is conqjared. The 
reading obtained in the field is simply a 
con^jarison of the soil redox potential at 
any given place with this calomel stan- 
dard. Figure 1 shows the comparison 
between a laboratory measurement and the 
field measurements using the redox probe. 
The left portion shows a copper electrode 
immersed in 1-molar sulfate solution 
separated from the zinc electrode 
in 1-molar zinc sulfate (ZnSO^) by a 
permeable membrane. In this case, 
the copper half-cell would have a 
potential of -0.3448 V, while the zinc 
half-cell would have 0.762 V. If the 
circuit between the zinc and copper 
electrodes were measured, the potential 
would be found to be 0.762 -(-0.3448) 
= 1.1068 V. 

The right portion shows the redox probe 
in place in the soil. In this case the 
calomel half-cell has a fixed voltage of 
-0.2415 V. The porous ceramic plug 
allows electrical conductance between 
this half-cell and the soil half-cell. 
The gold electrode is inert and allows 
measurements to be made without reacting 
with anything in the soil. The redox 
potential is thus measured between the 
soil and calomel half -cells. ' 



Several versions of the redox probe 
have been field tested. Although the 
probes have been of different shapes and 
sizes, the essential elements have been 
the same. 

The probe shell is constructed of an 
acrylic plastic, which is a good electri- 
cal insulator and is not chemically reac- 
tive in this use. A saturated calomel 
cell (Hg-Hg2Cl2) is used as the reference 
electrode, and a small gold disk is used 
as the inert electrode. Conduction 
between the calomel cell and the soil is 
provided by a fritted ceramic disk and a 
potassium chloride (KCl) solution salt 
bridge. A nonconducting handle is used 
for inserting and retrieving the probe 
from holes. 

Smaller probe versions have been con- 
structed of 1-1/2-in acrylic rod with 
single ceramic and gold electrodes for 
use where portable gasoline augers or 
hand augers were used for drilling the 
test holes (fig. 2). 

A larger, 4-in-diam probe has been con- 
structed for use with a tractor-mounted 
auger in order to increase the number of 
tests that could be performed in a day. 
This probe is large enough to accommo- 
date double ceramic and gold elec- 
trodes, which simplifies making probe 
contact with the bottom of the hole 
(fig. 3). 



1.1068V 




_! ZnS04 solution 

Potential 0.762V 



CuS04 solution 
Potential -0.3448V 



Laboratory 




Measured voltage 



Gold electrode 



Calomel half-cell 0,245V 
KCl solution for conductor 
Porous ceramic plug 



Field 



FIGURE 1. - Comparison between laboratory (left) and field (right) redox measurements. 




FIGURE 2, = Single=electrode, 1 = 1, 2=in=diam probe. 




FIGURE 3, = Double-electrode, 4-in=diam probe. 



The readings obtained from the probe 
are in millivolts. Any meter capable of 
reading -500 to +500 mV with a 2- to 4-mV 
resolution should work, as long as the 
input impedance is high (approximately 
mately 10' 2 ohms). In addition, the 



meter should be battery operated and tem- 
perature stable. Bureau investigators 
used a combination pH-millivolt meter, 
which proved to be lightweight and stable 
over long periods of time. 



FlfclLD CONSLDEtiATIONS FOR OBTAINING TEST DATA 



Because an absolute redox value cannot 
easily be assigned to a given geochemical 
anomaly owing to differences in reactant 
materials or soil makeup, a continuous 
traverse of readings must be made across 
both the suspected anomalous area as well 
as adjacent areas where the soil is unal- 
tered. In this manner, it is possible to 
establish a baseline against which the 
anomalous zone will be compared. 

Because of the daily and seasonal 
expansion and contraction of the soil due 
to temperature changes, there is a thin 
zone near the surface, from a few inches 
to 1 to 2 ft deep, that is exposed to the 
movement of atmospheric air. Redox anom- 
alies should not be expected to show up 
well in this zone of constant oxidation. 
Therefore, the test holes should be 2 to 
3 ft deep to get below this aereated 
zone. The Bureau's test holes were be- 
tween 4 and 6 ft deep where possible. 

The test-hole diameter should match the 
probe diameter as closely as practical. 
The hole should be cleaned as well as 
possible with the auger before inserting 
the probe to assure that the soil being 
measured is that representing the bottom 
of the hole, and not some that has fallen 
in from above. Also, the loose material 
has been mixed with air during augering 
and may not represent the undisturbed 
materials. The probe should be inserted 
as soon as possible after the hole has. 
been dug to prevent air circulation and 
excessive change in the redox potential 
at the bottom of the hole. The short ex- 
posure to air and the probe would affect 
a highly poised oxidation-reduction sys- 
tem such as a strong solution of chemi- 
cals, but due to the large number of ions 
involved per unit volume, the system 
would "bounce back" quickly. However, a 
soil-rock redox system is delicately 
poised, and owing to the much smaller 
number of reactive ions involved, the 
disturbance caused by exposure to the 
atmosphere and insertion of the probe 
takes a considerable amount of time to 
return to equilibrium. In the extremely 
dry soil encountered in southern Utah, 



this occasionally took as long as 40 to 
45 min, while in the damper soil encoun- 
tered in southwestern Colorado, equilib- 
rium often returned within 10 to 12 min. 

Rather than stating a fixed time for 
the soil to return to equilibrium, it is 
better to read the probe every 2 to 
5 min, and from these readings determine 
when the soil-probe system has stabi- 
lized. Generally, two or three readings 
on the flattening portion of an apparent 
asymptote can be considered stable 
for convenience sake and to expedite the 
survey (fig. 4), 

The auger and bits used in this study 
are shown in figure 5. This tractor- 
mounted auger uses a 4-in-diam auger and 
has the capability of using both soil and 
rock bits as shown. Figure 6 shows the 
probe emplaced in the hole with the 
millivoltmeter attached. A sponge- 
rubber collar is used to prevent probe 
movement in the hole and to minimize air 
circulation. 





Case where readings become identical 




a 

z ' 

o 

< 

UJ 


1 






• 





TIME — 





Case where readings become asymptotic 




C3 ( 

Z 
Q 
< 
UJ 

oc 










* 



TIME -* 



FIGURE 4. - Graphs showing data readings 
plotted against time. 




FIGURE 5- = Auger and bits used infield tests. 

EXPERIMENTAL FIELD RESULTS 

The following test cases illustrate the 
methods used in different environments. 
Some of the field notes regarding test 
conditions are included as a narrative 
description of the areas, as are soil and 
weather conditions that prevailed when 
the tests were conducted, which may help 
explain the results achieved. 

Case 1 

At the SUFCo Mine No. 1, located in 
south-central Utah, the elevations where 
work, was performed ranged from 7,500 to 
8,300 ft. Vegetation is medium to dense 
cedar with sparse grass. The area has 
moderate to heavy snow cover in winter 
and dry, arid summers. At the time of 
the field survey, in mid-July 1980, the 
soil was extremely dry and powdery to a 
depth of 4 to 5 ft. Daytime temperatures 
were typically in the low to middle nine- 
ties, and the relative humidity ranged 
from 10% to 15%. 




V'. 



FIGURE 6. = Field installation of probe. 

The target of interest at this site was 
a partially burned coal seam. Moderately 
good burn location control was available 
from company personnel and visual obser- 
vations, as well as existing mine maps. 
Tests were conducted at two areas: the 
first, site A, was adjacent to the mine 
portal and offices. The approximate 
depth to the coal seam was 40 ft at this 
test site. The second, site B, was about 
one-half mile west of the first, on top 



of a mesa and abou 
seam. Initially, it 
that a burn could be 
much overburden, bu 
sonnel reported that 
during cold weather 
mately 650 ft above 
under the rim of the 
was run. 



t 800 ft above the 
had not been thought 
reflected through so 

t reliable mine per- 
vents are observable 
at a point approxi- 
the seam on a slope 
mesa where the test 



Results, Site A. 

Measurements were made along the edge 
of a road starting over an area known to 
have been burned and extending past the 
portal, which was not burned. The trav- 
erse was 550 ft long, and measurements 
were taken at 50-ft intervals to detail 
the edges as closely as practical. In 
this area, it was suspected by company 
personnel that burn finger, or narrow 
burn zone, existed between the known burn 
and the known unburned coal near the por- 
tal area. This was based upon the exis t- 
ence of a small area of oxidized material 
in outcrop beyond the limits of the known 
burn. Hole depths were generally limited 



to 3 to 4 ft owing to difficult augering 
conditions, although in a few cases 
deeper holes were possible. 

The test site is shown in figure 7. 
The coal seam shows just to the left of 
the mine building, while the burned area 
is to the right of the switchback in the 
trail leading above the office. The red, 
oxidized zone above the burn is exposed 
more than 60 f t above the coal at some 
places. The graph of the field data 
(fig. 8) shows that the oxidized zone 
above the burn has a redox potential of 
140 to 150 mV, while that of the buff to 
gray-colored rock above the unburned coal 
is nominally about 100 mV. The rela- 
tively more oxidized rock above the burn 
also shows at about 200 ft and 300 to 
350 ft from the start of the traverse. 
The 200-ft readings correspond to the 
approximate location of the suspected 
burn finger. If this anomaly represents 
a burn, then it is also inferred from the 
data that a second finger may also exist 
at 300 to 350 ft. 




FIGURE 7. -' SUFCo site A showing coal seam and burn area. 



200 



100 



lu -100 - 



-200 



^^V-^ 




Jt 




^ 


4) 


1 (B 




n 1 


1 V 




0) 1 

c 1 


1 c 




a 1 


3 




o 


= 




c 


I 3 




- S 1 


1 C 




Kno 


Know 





- 



100 



200 300 400 

DISTANCE, ft 



500 



600 



FIGURE 8. - Field curveofSUFCo site A. 

Results, Site B 

This series of measurements started 
about 50 ft from the cliff edge at the 
top of the mesa over a known burn area 
and extended 2,100 ft westward to an area 
known to be unburned. The projected burn 
edge came from old mine maps and records, 
but as it is suspected that the burn is 
still active in this area, the edge may 
well be migrating westward from where 
shown. The depth to bedrock is generally 
less than 4 ft, and in many cases it is 
exposed at the surface. In those cases, 
where the sandstone was less than 2 ft 
deep, an attempt was made to auger 6 to 
12 in. into the rock with a rock bit in 
order to get below the zone of air oxida- 
tion. The soil in this area was ex- 
tremely dry, and times to soil-probe 
equilibrium were quite long, in some 
cases as much as 45 min. 

The graph of the field data from site B 
(fig. 9) is more difficult to interpret 
than site A and contains some apparent 
ambiguities. 

It had been hypothesized, before the 
coal-burn tests were started, that the 
fire gases would cause a reduction anom- 
aly over the burn, and that the unburned 
area would show a more oxidized baseline. 
However, the field data tend to show more 
oxidized readings over the burn areas, 
both at site A, where the tests were run 
under favorable conditions as far as 
proximity to the burn and known unburned 



areas were concerned, and also near the 
estimated burn front of site B. The data 
from site B, however, show a different 
behavior near where the tests were 
started that cannot be explained with 
current knowledge and the behavior of the 
data at site A. 

At the start of the traverse, a nega- 
tive potential exists. The potential 
goes positive within 75 ft and becomes 
still more positive until the estimated 
edge of the burn is reached, after which, 
an "unburned" baseline was reached. The 
area below the starting point of the 
traverse is the outcrop of the burned 
coal, and it is assumed that this is 
where the coal burned initially. If this 
is the case, the negative and more mildly 
oxidized readings should not occur if the 
same phenomena are responsible for the 
data at both sites A and B. The change 
from highly to less oxidized zones near 
the estimated fire front might be more 
difficult to interpret in an area where 
little was known about the burn location. 
Further studies will be made in the fu- 
ture to try to identify the various pro- 
cesses involved that caused such data as 
at site B. 

Case 2 

Bear Mine is located in southwestern 
Colorado at an elevation of 6,500 ft. 
Climate is wet in winter and spring, with 
dry, arid summers. The survey was run in 
a north-south-trending canyon parallel to 
the eastern edge of the mine. The soil 
materials are colluvium and valley fill 
and contain bits and pieces of sandstone, 
shale, and pieces of coal from the steep 
slopes and cliffs above. At no point in 
the survey did the drillholes encounter 
bedrock. The soil materials were con- 
sistently damp, and augering was very 
easy during field operations in mid-July 
1980. Two months earlier, an attempt was 
made to perform this survey, but free 
water was encountered in about 30% of the 
test holes, which caused erratic read- 
ings. The presence of free water flowing 
through the loosely consolidated materi- 
als apparently does not permanently alter 



10 



200 



> 
E 



u 

I- 
O 

o. 

X 

o 

o 

UJ 

flC 



■200- 



-400 



^^^/^-^T-v^ 


Estimated burned coal Estimated unburned coal 

1 

1 1 1 II 1 



300 600 900 1,200 

DISTANCE, ft 

FIGURE 9. = Field curve of SUFCo site B. 



1,500 1,800 2,100 



the redox characteristics of the soil, as 
the overall test results of this later 
experiment closely duplicated those run 
in earlier years. 

The target of interest in this area was 
a known concentration of methane caused 
by an accumulation at the crest of a 
small anticlinelike structure in the coal 
200 ft below the canyon floor. This 
methane had been a problem for the mine 
for many years. When working this area, 
it was not unusual, where entries were 
driven through the area, to have to leave 
the mine temporarily and use extra venti- 
lation before returning. During the 
1960's, the Bureau attempted to inter- 
cept the crest of the structure with a 
well to bleed off the methane in advance 
of future mining; but the well, based 
upon a theorized straight axis to the 
structure, was drilled to the south of 
the true location of the curved axis. 
Figure 10 is a 3-D computer model 
of the structure, which was generated 
from surveyed points in the coal seam. 
The test well and the anomaly loca- 
tion have been plotted onto this model. 
This methane anomaly had been used 
for other unpublished tests in 1970 and 



1977, and 
well known. 



its surface location 



was 



Measurements were made along a trail 
running up the floor of the canyon. The 
traverse was set up to cover an area 
approximately 500 ft each side of the 
anomaly. The test holes were nominally 4 
to 5 ft deep and were normally placed on 
the uphill side of the trail to stay away 
from disturbed materials where the trail 
had been cut into the hillside and filled 
towards the creek in the center of the 
canyon. The probe generally came to 
equilibrium within 10 min. The field 
data curve (fig. H), shows the peak of 
the anomaly about 500 to 550 ft down the 
canyon from the old Bureau of Mines test 
well. This is the same point where it 
was located in 1970 and 1977 during early 
to middle autumn, with much drier ground 
conditions. It may be seen from the fig- 
ure that the redox values are con- 
sistently higher than the previous data, 
but the anomaly is shown at the same 
location by both plots. The smoothness 
of the composite 1970-77 curves is due to 
the averaging effect of a much larger 
number of data points from these two se- 
ries of tests. 



11 




FIGURE 10- = 3-D computer model of Bear Mine. 



100 



> 
E 

_i 
< 

I- 
z 
liJ 

h- 
O 
Q. 

X 

o 

a 

lU 

tr 



100 



-200 




Composite 1970 
and 1977 tests 



200 400 600 800 

DISTANCE, ft 

FIGURE 11. - Field curve. Bear Mine site 



1,000 1,200 1,400 



12 



Case 3 

Hazel A Mine is Located in central Col- 
orado in the Front Range Mineral Belt. 
The area is typical of the Front Range, 
where elevations range from 7,000 to 
9,000 ft. The soil mantle is thin and 
quite rocky over an igneous-metamorphic 
con^slex of rock, which is predominantly 
granitic in character. The annual pre- 
cipitation is about 15 in, a considerable 
portion of which occurs as snow. This 
series of tests was run during September 
1980 when the soil conditions were dry. 
There were two known hydrothermal veins 
occurring on this property, and the redox 
traverses were run across these known 
veins. 

The composition of these veins is not 
known exactly by the Bureau. Float rock 
in the vicinity of the veins showed clay 
alteration and solution vugs as well as 
limonite and hematite deposition in and 
around the solutioned zones. Sulfide 
minerals observed in pieces of vein were 
sparse. From these observations, the 
oxidized anomalies found would seem to 
correspond to the character of the veins 
at the surface. Figure 12 is a plat of 
this test area. 

Test line A was a generally north-south 
line run across an east-west trending 
vein. The soil encountered was very 




Scale, ft 



Hazel A veins 
9/28/80 



rocky and extremely dry. Test holes 
averaged between 2 and 3 ft deep. The 
angering was quite difficult, and because 
of the dryness of the soil, it was nearly 
impossible to clean the hole with the 
auger. Generally, the bottom foot or so 
of the hole had to be cleared out by 
hand. 

The original traverse (line A) was made 
with test holes on 50-ft centers. The 
intermediate points near the vein were 
run after the anomaly was located. Fig- 
ure 13 shows the field curve for this 
site. An anomaly at 250 ft corresponds 
with the known vein, while the smaller 
perturbation in the curve between 75 
and 100 ft may be a possible paral- 
lel vein. The anomaly near 250 ft was 
later corroborated by a geologic plat 
of the area in possession of the mining 
company. 



JUU 


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200 


a. 








^^ 




100 


1 


1 




Line A 

1 







200 300 

DISTANCE, It 



FIGURE 13. ' Field curve, Hazel A Mine, Line A, 



250 




100 



100 200 

DISTANCE, ft 



300 



FIGURE 12. - Plat of Hazel A veins. 



FIGURE 14. - Field curve. Hazel AMine, Line B. 



Test line B was run about 850 ft west 
of line A across a generally north-south 
trending vein. This vein was quite easy 
to locate visually by a continuous line 
of prospect holes along 200 to 300 ft of 



13 



its length. The Bureau's test holes were 
35 ft apart, 2 to 3 ft deep in a decom- 



posed granite soil, 
figure 14, the vein 
the baseline. 



As may be seen from 
shows well against 



CONCLaSIONS 



From the results of this feasibility 
study, it appears that the redox method 
shows a potential as a means of locating 
geochemical phenomena associated with 
mining. 

The study left some questions unan- 
swered in regard to the interpretation of 
data from coal-burn areas. It is hoped 
that future investigations will concen- 
trate more heavily in this area, with 
a view to answering some of these 
questions. 

As it would be virtually impossible to 
predict a numeric value for any given 
redox phenomena in an area, a suitably 
long traverse must be run to assure that 
both unaltered ground as well as the 



anomalous ground are covered. This 
assures that an unaltered baseline is 
present with which to compare the anom- 
aly. In practice, this should present no 
great problem as the method will probably 
be used in areas where a general problem 
is suspected, rather than as a large- 
scale reconnaissance method. 

The Bureau's testing indicates that the 
smaller diameter probe would probably be 
preferable for most applications. This 
generally simplifies the drilling of the 
holes as a small, portable hand auger may 
be used rather than a tractor-mounted 
rig, which is easier in rough topography. 
In addition, the smaller probe seems less 
susceptible to rough handling than the 
larger probes. 



14 



APPENDIX 



The construction of the redox probe is 
fairly simple. The probe shell requires 
some light machining, but the dimensions 
are not critical. 

In effect, the probe shell is simply an 
electrically and chemically inert device 
to hold the calomel electrode and the 
gold electrode in a constant relationship 
to one another. Its other function is to 
allow the electrodes to be lowered into a 
hole and be pressed against the soil in 
the bottom of the hole. 

Because the potassium chloride (KCl) is 
an electrical conductor, the chamber con- 
taining the calomel cell and this solu- 
tion must have a watertight seal at the 
top to prevent an internal short circuit 
between the chamber and the gold elec- 
trode. The last element to the probe is 
a tube through which the KCl solution can 
be replenished when necessary. The 
Bureau used a small-diameter hole drilled 



into the calomel cell chamber and fitted 
with a tight-fitting, flush rubber stop- 
per to prevent dirt intrusion. 

Construction Notes 



1. The general shape of the probe tip 
as shown on the shop drawings (figs. A-1 
and A-2) has been found to be superior to 
flat or conical shapes for making contact 
with the bottom of the hole. 

2. The dimensions of the calomel cell 
chamber must be such that the cell can be 
sealed tightly at the top to prevent 
solution leakage if the probe is tipped 
or inverted. 

3. Both the ceramic and the gold elec- 
trodes should be sanded to conform 
smoothly with the probe shell to prevent 
dirt entrapment at this point. This also 
allows the probe shell to protect the 
ceramic from chipping or breaking. 



Ceramic frit 




Calomel cell chamber 



Ox ,x OC-<^0s.X < 



'/s-in hole, V32-in deep for gold electrode 




PROBE 



To fit connector 
(cemented) 




To fit 1V2-ln-0D 
handle (5 ft long) 



CONNECTOR 

FIGURE A-1. - Construction details of single-electrode probe. 



15 




Ceramic frit .^s/'l-- 

\ \ Calomel cell / / i 

.\ \ \ / chamber / / | 

\\ \4 "^"-"-"^. 

^\ ^^ ^ A____A^ 



10 threads 
per inch 




Gold electrode 



8- 



PROBE BODY 



^^I^^X" 



10 threads 
per inch 



Drill V. 




PROBE CONNECTOR 

FIGURE A-2. - Construction details of double-electrode probe. 



4. Bot:h the ceramic and gold elec- 
trodes should be mounted to the probe 
shell with silicone rubber to prevent ex- 
cessive solution leakage or dirt and 
water intrusion into the probe. 

5. The mounting lug on the probe 
should be lathed to a snug fit to what- 
ever type handle is to be en5)loyed. Nor- 
mally, a rigid plastic or PVC tube works 
satisfactorily for a handle. 

Parts List 

Fritted Ceramic Material 

Coors P 1/2 BC (A fritted ceramic with a 
very slow flow rate) (Some calomel cells 
are equipped with a fritted ceramic, 
which may be removed and used in the 
probe. ) 

Calomel Cells (Capable of disassembly and 
insertion into probe) 

<rU.S GOVERNMENT PRINTING OFFICE: 1982 - 505 - 002/81 



Van Waters and Rogers 

Beckman 

Ingold 
Probe Shell 



34106-148 

34119-183 

40459 

34105-666 

5502-01 



Acrylic or polycarbonate rod of suitable 
diameter. 

Note: 

The above parts are listed as an aid to 
procuring the necessary con5)onents for a 
probe, and in no way should be construed 
as an endorsement by the Bureau of Mines. 
Other similar materials are available 
from chemical supply houses, but an ex- 
haustive search has not been made by the 
authors. 



INT.-BU.OF MINES,PGH.,P A. 26329 






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